1. Field of the Invention
The present invention relates to a photodiode and, in particular, to fast photodiodes as are, for example, utilized in optical memories and in optical communications.
2. Description of the Related Art
Photodiodes are applied in various fields. Photodiodes are, for example, employed in X-ray detectors, camera sensors, triangulation sensors, optocouplers, monitoring devices, optical memories and optical transmission systems. In particular in the last-mentioned fields of application, photodiodes enabling signal processing in the megahertz to gigahertz region are required.
A photodiode utilizes the electrical field of a pn junction to capture charge carriers generated by light or to separate electron/hole pairs. The resulting photocurrent is composed of a drift portion and a diffusion portion. The. drift portion is formed by charge carriers generated in the space-charge zone of the pn junction and thus detected directly via the photocurrent. The drift portion of the photocurrent generated in this way has a bandwidth in the gigahertz region.
The diffusion current is formed by charge carriers generated outside the space-charge zone. After being generated, the charge carriers diffuse through the substrate due to a lacking field impact without an electromotive force and are detected at a time interval, offset from the generation, depending on the diffusion length. The bandwidth of this diffusion portion is considerably smaller than that of the drift current portion, namely smaller than about 10 MHz.
The percentage of the drift portion compared to the entire photocurrent approaches zero for large wavelengths. With an increasing wavelength, the bandwidth of the photocurrent consequently corresponds to the bandwidth of the diffusion portion. In contrast, the sensitivity of photodiodes at first increases with an increasing wavelength. The reason for this is that the penetration depth of light on the one hand increases with an increasing wavelength and that the recombination of charge carriers at the surface on the other hand is higher than in charges generated in deeper regions. The result is an increase in sensitivity or responsivity of the photodiodes with an increasing wavelength Starting at a certain wavelength at about 650 nm, the sensitivity of the photodiode, however, decreases with further increasing wavelengths since, due to the still greater penetration depth of long-wave light, charge carriers are generated in even deeper regions and the charge carriers generated in even deeper regions and diffusing towards the space-charge zone of the photodiode can no longer reach the space-charge zone of the photodiode before recombining.
Up to now, ways of addressing the problem of a limited bandwidth in systems requiring a photodiode having a bandwidth of several megahertz have been to fall back on discrete photodiodes produced by means of expensive special processes. Apart from the expenses for the external devices, this causes further expenses for example due to additional casing pins, bond pads, high-area and high-current input/output interfaces and additional printed circuit board area.
An alternative solution allowing fast photodiodes without modifying a process is described by Cathleen Rooman et al in “Asynchronous 250-Mb/s Optical Receivers with Integrated Detector in Standard CMOS Technology for Optocopler Applications,” IEEE Journal of Solid State Circuits, vol. 35, No. 7, July 2000. In order to optimize speed without modifying a process, half of the diode area is used to detect the diffusion current. Additional circuit blocks calculating the diffusion portion from the photocurrent comprising a diffusion and a drift current portion are provided for this. The disadvantage of this procedure is that about half of the light power is employed for detecting the diffusion current. Furthermore, there are several non-amplifying devices in the signal path, which in turn means a considerable deterioration of the noise features of the read-out. In particular in communications and optical memory systems, noise is a critical parameter so that the procedure used there can only be employed in such systems under certain circumstances.
Another disadvantage of prior-art photodiodes generated on the process level is that they exhibit, as has been described before, a sensitivity characteristic curve which depending on the wavelength is not constant. A linearization of the sensitivity could, for example, be desired for reasons of compatibility in optical memory systems in which data is read out from data carriers, which can be archived, in different devices by means of light sources having different wavelengths. Up to now, such a linearization has only been achieved on the side of signal processing by means of complex circuit blocks, such as, for example, digital signal processors.